Set aside 1/4 cup sliced scallions. Pulse cilantro, garlic, citrus zests, citrus juices, soy sauce, oil, salt, and remaining scallions in a food processor or blender until a coarse purée forms.
et aside 1/4 cup marinade; place remaining marinade in a large resealable plastic bag. Add chicken, seal bag, and turn to coat.
Chill at least 20 minutes.
Preheat broiler. Remove chicken from marinade and place, skin side down, on a foil-lined broilerproof baking sheet; discard marinade. Broil chicken until lightly browned, about 5 minutes.
Turn; continue to broil until cooked through and an instantread thermometer inserted into the thickest part of thigh registers 165°F, 12-15 minutes longer. Serve chicken with reserved marinade and scallions.
DO AHEAD: Chicken can be marinated 2 hours ahead. Keep chilled.